Revise Rust collections documentation

[dilroop-us]

Updated the introduction and added runnable examples for Rust collections to improve clarity and consistency.
Also included concise usage notes for each collection type to help readers choose the right one.

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Description

Updated the collections.md entry under Rust by:

• refining the introductory explanation
• adding full examples for all major collection types
• adding a comparison/usage guide to explain when to use which collection
• improving consistency and readability without changing the technical meaning

Issue Solved

N/A — documentation improvement

Type of Change

• Editing an existing entry (clarity and completeness)
• Updating the documentation

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I am pushing from a new branch named after the change.
• N/A for issue linking (no linked issue).

[CLAassistant]

All committers have signed the CLA.